1. Technical Field of the Invention
The present invention relates generally to electric rotating machines that include a multi-phase stator coil. More particularly, the invention relates to an electric rotating machine that has an improved stator coil arrangement for reducing magnetic noise and torque ripple and is drivable with only a single three-phase inverter.
2. Description of the Related Art
A conventional three-phase electric rotating machine includes a stator and a rotor. The stator includes a hollow cylindrical stator core and a three-phase stator coil wound around the stator core. The rotor has a plurality of magnetic poles formed by permanent magnets. The magnetic poles are so arranged that the polarities thereof alternate between north and south in the circumferential direction of the stator core.
In such an electric rotating machine, magnetic noise and torque ripple are generally caused by the overlapping of harmonic components with sine-wave drive currents supplied to the phase windings of the stator coil. Further, in terms of magnetic flux, the main cause of magnetic noise and torque ripple is the third harmonic component of magnetic flux. Moreover, electromagnetic force is proportional to the square of magnetic flux. Therefore, in terms of electromagnetic force, the main cause of magnetic noise and torque ripple is the sixth harmonic component of electromagnetic force.
Japanese Patent Application Publication No. 2008-5603 discloses a synchronous machine in which: the stator coil is comprised of first and second winding groups; and the ratio of the number of magnetic poles of the rotor to the number of slots of the stator is equal to 10:12 or 14:12. The first winding group includes a plurality of windings each of which corresponds to one of U, V, and W phases. The windings of the first winding group are Δ-connected. On the other hand, the second winding group includes a plurality of windings each of which corresponds to one of X, Y, and Z phases. The windings of the second winding group are also Δ-connected. Further, the first and second winding groups are wound around the stator core in a concentrated winding manner so that: the windings of the first winding group are alternately arranged with those of the second winding group in the circumferential direction of the stator core; and each circumferentially-adjacent pair of one of the windings of the first winding group and one of the windings of the second winding group are wound in opposite directions. Consequently, the phase difference in electrical angle between each circumferentially-adjacent pair of one of the windings of the first winding group and one of the windings of the second winding group is equal to π/6. Moreover, the phase difference of π/6 is equal to half the wavelength (i.e., π/3) of the sixth harmonic components of the electromagnetic forces created by the windings of the first and second winding groups. Therefore, the sixth harmonic components of the electromagnetic forces created by the windings of the first winding group can be offset by those of the electromagnetic forces created by the windings of the second winding group. As a result, when the synchronous machine is used in a mechanical power generation system to function as an electric motor, the total magnetic noise and torque ripple generated in the synchronous machine can be reduced.
However, with the stator coil arrangement disclosed in the above patent document, it is necessary to employ two three-phase inverters to supply first and second three-phase AC currents, which are different in phase by π/6, respectively to the Δ-connected windings of the first winding group and the Δ-connected windings of the second winding group. Consequently, with the use of the two three-phase inverters, the scale, cost, and complexity of the entire mechanical power generation system will be increased.